xvid does not match bitrate at all (2pass)

[TactX]

Hi folks,

after years of absence (some of you might still know me) from encoding stuff i have tried to convert a simple DVD with a bit of a problem. I was not really sure which subforum to post this to, but the long absence qualifies me as newbie for sure ;)

I tried to convert a dumped stream with mencoder and xvid as codec. First pass:
mencoder stream.dump -vf crop=720:432:0:72,scale=720:304 -ovc xvid -xvidencopts bvhq=1:chroma_opt:quant_type=mpeg:bitrate=1270:pass=1 -oac copy -o /dev/null

Second pass:
mencoder stream.dump -vf crop=720:432:0:72,scale=720:304 -ovc xvid -xvidencopts bvhq=1:chroma_opt:quant_type=mpeg:bitrate=1270:pass=2 -oac mp3lame -lameopts br=128:cbr:vol=6 -o output.avi

The resulting video is ok but the targeted bitrate does not match at all. It gives me only 821.257 kbit/s (average) while i've set it up to obviously use 1270 kbit/s.

While the result looks not to shabby i might live with that, but the question remains: Why the hell is that? What have i done wrong?

Thanks in advance

[Dark Shikari]

If Xvid is unable to reach your target bitrate while abiding by the default quantizer restriction (qpmin=2), the result will be lower than your target.

[TactX]

Thanks for your answer.

So i should leave it just as it is (with propably sane default settings)?